1. Field of the Invention
The present invention generally relates to photo-sensor arrays and more particularly to a unique three-dimensional pixel structure that significantly improves pixel packing density.
2. Description of the Related Art
Semiconductor photosensors have been found in a wide variety of applications. These include position measurement, CMOS imagine sensors, motion detector, image capturing and velocity measurement. One key application of these devices however is for optical-fiber communication.
The basic photo sensing mechanisms, as summrarized by S. M. Sze in the text book of Physics of Semiconductor Devices, p. 743 (incorporated herein by reference), are: (1) carrier generation by incident light, (2) carrier transport and/or multiplication by some sort of current-gain devices, and (3) interaction of current and IC circuits to provide output signals. A well-designed photo-sensor provides high sensitivity at operating wavelengths, high response speed, and minimum noise. It is desirable that photo-sensor chips be small in size, reliable under operating conditions, and operated at low power.
From a device aspect, photo-sensors can be presented in many different types, such as p-i-n diode, p-n diode, metal semiconductor diode, metal-i-n diode, etc. In general, p-n diodes have a lower response speed than p-i-n diodes (described in greater detail below). This is because the generated photocurrent consists of large portions of diffusion current and small portions of drift current due to thin depletion region. At long wavelengths, the required absorption depth becomes very long which causes performance of p-n diodes to degrade further.
One of the reasons for the increased performance of p-i-n diodes is that they include a depletion region (or the intrinsic layer) which has a thickness that allows p-i-n diodes to be tailored to optimize quantum efficiency and frequency response. The basic photosensing mechanism of a p-i-n diode has light absorption in the depletion (or i-layer) region that produces hole-electron pairs which will be separated by an applied electric field. The diode is reverse biased, so that electron xe2x80x9cholesxe2x80x9d drift to the p terminal, which is tied to ground, while electrons drift to the n terminal, which is tied to a positive voltage. This results in higher current flow in the external circuit than that of the p-n diode sensors due to large drift space.
If metal is used to form photosensors, usually it has to be very thin (10 to 20 nm) so that it is semi-transparent to the incident light. In general, metal is also highly reflective and an anti-reflective coating (e.g., 50 nm of ZnS) is necessary to enhance quantum efficiency.
Another application for photosensors is use as an image sensor. Complementary metal oxide semiconductor (CMOS) image sensors have advantages such as low-cost, low-power, and a high level of integration. CMOS image sensor can be used in digital cameras or devices such as motion detectors. In general, each pixel of CMOS image sensor comprises ⅕ circuit area, and ⅘ diode area. Further, in order to ensure sufficient total photon flux, conventional two-dimensional p-n photosensors are inherently designed with large spacing. Therefore, conventional CMOS image sensors have relatively poor pixel density and there is a need to increase the pixel density.
In view of the foregoing and other problems, disadvantages, and drawbacks of the conventional photo-sensor arrays, the present invention has been devised, and it is an object of the present invention to provide a structure and method for boosting pixel density of a photo-sensor array. Pixel density is defined as number of pixels which can be packed in a unit chip area. The second object of the invention is to provide a unique three-dimensional pixel structure so that pixel packing density can be significantly improved. Another object is to provide optimize the sensor""s quantum efficiency. A further object is to use a conductive polymer to fill in the gaps in the sensor array and to improve reverse biasing of the p terminal of each photo diode without blocking the light.
In order to attain the object(s) suggested above, there is provided, according to one aspect of the invention a photodiode array comprising a plurality of photodiode cores, light sensing sidewalls along an exterior of the cores, logic circuitry above the cores, trenches separating the cores, and a transparent material in the trenches.
With the invention, the sidewalls are perpendicular to the surface of the photodiode that receives incident light. The light sensing sidewalls comprise a junction region that causes electron transfer when struck with light. The sidewalls comprise four vertical sidewalls. The core comprises a n+ core and the sidewalls comprise p+ sidewalls. The logic circuitry blocks light from the core.
More specifically, the island pixels have an n+ core having a cube shape, an intrinsic layer surrounding sides of the n+ core, a p+ layer surrounding sides of the intrinsic layer and at least one transistor above the n+ core. There is also an n-well between and connecting the n+ core and the transistor. The p+ layer comprises a p-type doped layer having a low doping concentration and the n+ core comprises an n-type low doped layer.
An anti-reflective coating surrounds the sides of the p+ layer and a transparent material is adjacent the anti-reflective coating. There are also wiring levels above the transistor and the transparent regions. The wiring levels include transparent regions above the transparent material.
The light absorption sidewall regions are perpendicular to the surface of the pixel that receives the incident light, while conventional light absorption regions are made parallel to the pixel surface. With the invention, the upper surface of the island maintains the necessary logic circuitry and the upper surface is not a region where substantial amounts of light are absorbed. To the contrary, with the invention, the openings surrounding each pixel island allow angled light beams to directly strike the vertical light absorption surfaces. Further, light beams that are directly perpendicular to the upper surface of the array of are reflected from the trenches surrounding each of the islands to one of the adjacent vertical light absorption regions. Also, the light beams will produce multiple internal reflections inside the pixel island, which also improves the diode quantum efficiency.
Since the light absorption regions are perpendicular to the upper surface of the array, they do not consume any substantial amount of the two-dimensional area of the upper surface of the array. Only logic circuitry and the trenches between the pixel islands consume two-dimensional area of the upper surface of the array. Thus, the inventive three-dimensional photo-diode island design realizes an increase in sensor packing density.